220.127.116.11.2 HTTP Error 451
If the client is attempting to connect to the wrong server (that is, a server that cannot access the user's mailbox), or if there is a more efficient server to use to reach the user's mailbox, then a 451 Redirect error is returned.
The error returned by the wrong server resembles the following:
OPTIONS /Microsoft-Server-ActiveSync Content-Type: application/vnd.ms-sync.wbxml MS-ASProtocolVersion: 14.0 HTTP/1.1 451 Date: Tue, 08 Dec 2009 19:43:24 GMT Server: Microsoft-IIS/7.0 X-Powered-By: ASP.NET X-AspNet-Version: 2.0.50727 X-MS-Location: https://mail.contoso.com/Microsoft-Server-ActiveSync Cache-Control: private Content-Length: 0
If an X-MS-Location header is present in the response, all subsequent requests SHOULD use the URL specified within the X-MS-Location header. If the server does not provide an X-MS-Location header in its response to the client, then the full Autodiscover command process is followed, as specified in [MS-ASCMD].